The present invention relates to a system and corresponding process for separating a gaseous component from a gas mixture employing a combination of at least one regenerator and at least one ceramic membrane to produce a high purity gaseous component stream. In particular, one preferred embodiment of this invention relates to an energy efficient, cost effective process for producing a high purity oxygen stream from an air stream or from a high purity nitrogen or argon gas stream that contains an oxygen impurity.
The separation of oxygen from an oxygen-containing gaseous stream is a process step in a number of commercially significant manufacturing operations. One method of oxygen separation utilizes a mixed conductor ceramic membrane material (also sometimes referred to as an xe2x80x9coxygen ion transport membranexe2x80x9d or xe2x80x9c(OTM)xe2x80x9d or an xe2x80x9cionic/mixed conductor membrane unitxe2x80x9d). Oxygen ions and electrons are selectively transported through this non-porous ceramic membrane material that is impervious to other species.
Suitable ceramics include mixed conductor perovskites and dual phase metal-metal oxide combinations. Exemplary ceramic compositions are disclosed in U.S. Pat. No. 5,342,431 (Anderson et al.); U.S. Pat. No. 5,648,304 (Mazanec et al.); U.S. Pat. No. 5,702,999 (Mazanec et al.); U.S. Pat. No. 5,712,220 (Carolan et al.); and U.S. Pat. No. 5,733,435 (Prasad et al.). All of these references are incorporated herein by reference in their entireties.
Ceramic membranes formed from solid electrolytes and mixed conducting oxides typically exhibit the property of oxygen selectivity. xe2x80x9cOxygen selectivityxe2x80x9d means that only oxygen ions are transported across the membrane with the exclusion of other elements and ions. Particular advantageous solid electrolyte ceramic membranes are made from inorganic oxides, typically containing calcium- or yttrium-stabilized zirconium or analogous oxides having a fluorite or perovskite structure. Use of such membranes in gas purification applications is described in U.S. Pat. No. 5,733,069 entitled xe2x80x9cReactive Purge For Solid Electrolyte Membrane Gas Separationxe2x80x9d by Prasad et al., which is also incorporated herein by reference in its entirety.
Ceramic membrane materials have the ability to transport oxygen ions and electrons at the prevailing oxygen partial pressure in a temperature range of from 450xc2x0 C. to about 1200xc2x0 C. when a chemical potential difference is maintained across the membrane element. This chemical potential difference is established by maintaining a positive ratio of oxygen partial pressures or an electrical gradient across the ion transport membrane. The oxygen partial pressure (Po2) or electrical gradient is maintained at a higher value on the cathode side of the membrane, that side exposed to the oxygen-containing gas, than on the anode side, where the transported oxygen is recovered or used.
Ceramic membranes used in air separation for the production of high purity oxygen require the feed air to be raised to an elevated temperature (800xc2x0 C. to 1000xc2x0 C.) before being introduced into a membrane unit.
A number of existing systems for air separation using ceramic membranes heated the feed air to the operating temperature of the membrane by combustion. Typically, a fuel gas was mixed with the feed air and the mixture was burned. The heat of combustion is sufficient to raise the temperature of the entire air stream. Alternatively, heat transfer surface areas within the ceramic membrane unit itself have been used, or separate heat exchangers used, to heat the feed air stream by recovering energy from the streams leaving the membrane unit or an associated gas turbine unit.
The use of combustion to heat the feed air has several disadvantages. First, a portion of the oxygen in the feed air is consumed, thus reducing the partial pressure driving force for the membrane unit. Second, combustion adds undesirably CO2 and water vapor into the feed air stream. Third, some means of recovering thermal and pressure energy from the hot waste nitrogen stream must be included in the process. The flow rate of the waste nitrogen stream is about 85% of that of the original feed air. Although it would seem to be a simple matter to expand the hot waste nitrogen in a gas turbine, the operating temperature of the ceramic membrane unit and therefore the temperature of the hot waste gas is above practical limits for such turbines.
The use of a heat transfer surface area within the membrane to heat the feed gas also has disadvantages. For example, it greatly increases the cost of the membrane construction and operation and for most commercial applications is prohibitively too expensive. Also, the use of conventional heat exchangers is generally too expensive if the goal is to raise the temperatures of the feed gas more than about 200-300xc2x0 C. because of the inherent relatively poor heat transfer in such equipment. This extra cost can increase greatly when very high temperatures are required because of the need for exotic materials of construction to withstand the combination of pressure and intense heat.
Specific examples of prior art systems for heating the feed air stream to a membrane system are described in the following references:
European Patent Application No. 658,366 (Kang et al.) describes a method for the recovery of oxygen from air in which a high temperature ion transport membrane system is integrated with a combustion turbine system. Coproduction of oxygen and electric power is achieved in one embodiment by integrating a power generation system with an ion transport membrane system. The design performance of the gas turbine in the power generation system is maintained by controlled water injection into the membrane non-permeate stream which is introduced into the gas turbine combustor. Alternatively, makeup air is added to the membrane feed to maintain the performance of the gas turbine. NOx formation is reduced by introducing the oxygen-depleted non-permeate from the membrane system to the gas turbine combustor. While this published European patent application employs heat exchangers and water injection for temperature control, it does not mention the use of regenerators for temperature control.
U.S. Pat. No. 5,035,727 (Chen) describes a process for recovering oxygen from an externally-fired power generating gas turbine system in which a hot compressed air stream is first passed over the feed side of a solid electrolyte oxygen selective membrane while a positive oxygen ionic potential is maintained on the membrane to separate oxygen from the stream. The oxygen is removed from the permeate side of the membrane and the waste gas stream is expanded through the turbine expander to generate power. The patent teaches the use of heat exchangers to control the temperature of the process streams. The patent does not mention the use of regenerators to control temperature.
U.S. Pat. No. 5,174,866 (Chen et al.) teaches a process for extracting high purity oxygen from the exhaust stream of a gas turbine generator by passing the turbine exhaust over the feed side of a solid electrolyte membrane selective to the permeation of oxygen, separating the oxygen from the exhaust stream. The oxygen product stream is then removed from the permeate side of the membrane. The reject exhaust stream is then passed through a power generating turbine to recover net power. This patent employs heat exchangers in the system instead of regenerators.
U.S. Pat. No. 5,797,997 (Noreen) describes a thermophotovoltaic (TPV) system that converts air/fuel mixtures into oxygen and electricity for use in a variety of applications. The TPV system efficiently generates oxygen and electric power through the combustion of fossil fuels in air with little or no nitrogen oxides or other undesirable combustion by-products. Combustion temperatures are kept at about 1700xc2x0 C. or lower while burning a reactant having an air/fuel ratio of greater than about 3:1. Heat from combustion products can be recycled and recovered without concern for excessive increases in combustion temperature and subsequent combustor/emitter degradation. The generated oxygen may be harvested for commercial use as well as for introduction back into the TPV system. An insulated housing containing a porous ceramic matrix is used having a combustion zone for supporting combustion of a reactant mixture and for emitting radiant energy in response to the combustion. A heat recovery system is used to recover the heat of combustion and for preheating the reactant mixture. A means is provided for transferring the generated electricity to the membrane for separating the oxygen. The heat recovery system is integrated with the porous ceramic membrane. This patent does not use separate regenerators for preheating and heat recovery in the system.
U.S. Pat. No. 5,852,925 (Prasad et al.) teaches a method of producing oxygen and generating power where (1) a compressed oxygen-containing gas stream is first heated in a heat-exchanger to preheat the gas stream to about 300xc2x0 C. to about 800xc2x0 C., then enters a combustor, where it is combusted with fuel. The combusted, compressed gas stream is then contacted with a solid electrolyte membrane to produce an oxygen-depleted, compressed gas stream and pure oxygen stream. The oxygen-depleted gas stream is further combusted and then expanded in a gas turbine to produce power. The patent does not mention the use of regenerators to control temperature.
The proposed invention addresses the problems of prior art processes where combustion was used to heat the feed gas to the membrane or where a heat transfer surface area is built into the membrane or where conventional heat exchangers are employed.
Separately, regenerators have been used in the steel and cryogenic air separation industry. Examples of prior regenerator applications include the following:
xe2x80x9cThe Making, Shaping and Treating of Steelxe2x80x9d, 10th Edition, US Steel, AISE, pp. 557-585, discusses the use of regenerators operating at high temperature to preheat the blast air for use in steel-making blast furnaces.
U.S. Pat. No. 2,671,324 (Trumpler) teaches the use of regenerators in a cryogenic air separator plant.
These prior art applications of regenerators do not contemplate or suggest their use with selective ceramic membranes of any type.
Therefore, one aspect of the present invention is directed to a system for separating a high purity gaseous component from a gaseous mixture, comprising:
a feed gas stream comprising the gaseous mixture;
at least one regenerator capable of heating at least a portion of the feed gas stream, and
at least one selective ceramic membrane capable of separating a gaseous component from the gaseous mixture in the heated feed gas stream and thereby forming a high purity gaseous component.
Another aspect of the present invention is directed to a process for separating a high purity gaseous component from a gaseous mixture, comprising:
passing at least a portion of a feed gas stream containing the gaseous mixture through at least one regenerator to form a heated feed gas stream; and
passing the heated feed gas stream through at least one selective ceramic membrane to separate a gaseous component from the heated feed gas stream; thereby forming a high purity gaseous component and a waste gas stream.
One preferred embodiment of the present inventive system and process is the separation of a high purity oxygen stream from an air stream using the combination of at least one regenerator to heat the air stream followed by passing the heated air stream through a mixed conductor ceramic membrane to separate the oxygen from other gaseous components of the air stream.
Another preferred embodiment of the instant invention uses regenerators followed by an optional heating means to further raise the feed air temperature to the desired level for a mixed conductor ceramic membrane. The optional heating means includes either combustion of the heated gas stream with a fuel gas such as natural gas or combustion of the feed gas stream with a carbon monoxide-hydrogen mixture (syngas) or by electrical heating.
Electrical heating is preferred since no combustion takes place, thus the partial pressure driving force for the membrane is not reduced and no combustion by-products are introduced to the feed air stream.
In still another preferred embodiment, the thermal energy of the hot waste gas is recovered by using it to preheat the feed air stream. This is accomplished by using the hot waste gas to heat the ballast material contained in a regenerator before the feed air is permitted to pass through the regenerator. The pressure energy and any remaining thermal energy of the waste nitrogen stream is recovered by expanding it using a conventional hot gas expander downstream of the regenerator.